1. Field of the Invention
The present invention relates to a rotational position detector for a robot, and particularly, to a rotational position detector for detecting the rotational positions of each arm or a wrist member of the robot which is capable of multiple rotations around the same axis.
2. Description of the Prior Art
The conventional detector has such a construction as shown in FIG. 3, wherein a motor 3 rotating a wrist 2 through a reduction gear 1 is connected to a resolver 5 through a reduction gear 4.
As the resolver 5, there is used a so-called absolute value detector for outputting the magnitude of a rotational angle in terms of an absolute value signal by each rotation of a detection shaft.
The motor 3 is mounted with another resolver 6 (so-called precise resolver) for precisely detecting the rotational positions of the output shaft of the motor 3.
When the wrist 2 is rotated by the motor 3, the rotational position of the wrist 2 is detected based on an output signal from the resolver 5.
In the aforesaid conventional detector, even when the resolver 5 is rotated by a large number of times, the rotational positions can be continuously detected by counting the rotational number of the resolver 5 or the like. However, for example, when the power is turned off once due to a trouble and is turned on again, there occurs one cannot correctly detect the position of the wrist 2 according to an output signal from the resolver 5.
This is because of the fact that, as shown in FIG. 4, for the same signal values from the resolver 5 to the wrist 2 is capable of multiple, the rotational positions which differ from each other, shown, as the points A, B and C, depending on the total angle of the wrist 2. Therefore, the rotational angle of the wrist 2 must be limited so that the rotation of the resolver 5 can be kept within one turn for realizing an absolute position detection mechanism in the aforesaid detector.